This invention relates to novel filled polypropylene compositions characterized by good physical properties and high ignition resistance.
Polypropylene, which is readily available at a reasonable cost, has found many industrial uses because of its desirable physical properties such as ease of fabrication by all conventional methods; high melting point of stereoregular, e.g., isotactic, polypropylene; and compatibility with many other commercial resins, which permits a large number of blends having specific properties. Isotactic polypropylene is inherently brittle, that is, it has a low impact resistance. Brittleness can be reduced either by copolymerizing propylene with ethylene to form block copolymers or by blending homopolypropylene with rubber, for example with EPM or EPDM elastomers. These elastomers are well known to the art. They are copolymers of ethylene with propylene and in the latter case, one or more diene monomers, at least one of which has double bonds that polymerize at different rates.
Polypropylene-based compositions have found many applications, including automobile parts, electric wire insulation and housings for appliances and instruments. Some applications demand a composition that is ignition-resistant or fire-retardant. It is known in the art that polypropylene can be fire-retarded with various combinations of halogenated organic compounds and antimony oxide, and that homopolypropylene is easier to fire-retard than are polypropylene/polyethylene block copolymers or polypropylene/elastomer blends. A disadvantage of such halogen-containing fire-retarded compositions is that they evolve toxic and corrosive fumes when exposed to flame or strong heat.
A halogen-free fire retardant for polyolefins is described in U.S. Pat. No. 4,072,643 to Bost. This material, described as `intumescent fire retardant` (hereinafter, sometimes abbreviated IFR) - made by heating a mixture of a polyol, a phosphorus acid, and melamine - imparted a high degree of ignition resistance to polyolefin compositions as measured by the Underwriters Laboratories UL-94 test. Most sample compositions reported in the patent have a rating of V-O (the highest and most desirable rating) and a few of V-1. It has now been found, however, that IFR does not adequately fire-retard polypropylene-based compositions when various conventional mineral fillers are included in the composition. Addition of a filler is frequently desirable in order to reduce cost, increase stiffness, enhance opacity, or impart other desired properties to the composition. Accordingly, it appeared desirable to provide a filler which would not adversely affect the fire-retardant action of IFR.